Syphilis is a bacterial sexually transmitted infection which causes painless sores on the genital, mouth or rectum. It is not easily diagnosed and can go undetected and so treating it early is important as it can cause serious damage to other organs in the body like the brain and heart. Syphilis can only be transmitted through direct contact with syphilis sores.
Sources:
https://www.cdc.gov/std/syphilis/stdfact-syphilis.htm
https://www.mayoclinic.org/diseases-conditions/syphilis/symptoms-causes/syc-20351756
https://medlineplus.gov/syphilis.html
https://www.webmd.com/sexual-conditions/syphilis#1
https://www.healthline.com/health/std/syphilis#what-is-syphilis